Sondes are small, self contained transmitters used to trace the path of non-metallic pipes and locate blockage in a line. Sondes are inserted into a pipe or duct by means of a push rod and the location found by means of a receiver.

Actually I have done the same kind of thing using a valve solenoid on the end of a fish tape. Just hook up two long wires to the solenoid, tape it to the fish tape (or a small pipe), and shove it down the pipe you are tracking. Hook it up to your 521 locator and follow the "hot spot". I've found the location of damaged sleeves and pipes underneath concrete and asphalt using this method.

That's a great idea I never thought of. There are a few situations where I had to have roto rotor out with their camera, the "solenoid on a stick" would have worked. I probably would not have done this on the job yesterday. Public sewer line and all.